A cathode for a metal/air battery is generally used in a button battery. It has the shape of a disc situated under aeriation holes provided in the casing. An air diffuser paper is generally positioned between the casing and the cathode. In contrast to other types of batteries, the cathode in a metal/air battery should store only the quantity of electroactive material (typically oxygen) which is necessary for the immediate requirements of the battery, the rest being replaced bit by bit from the exterior, through the aeriation holes. For this reason, the cathode can be produced from a very thin strip, allowing the majority of the volume of the battery available for the anode (for example zinc in the case of a zinc/air battery).
The cathode of the metal/air battery is formed from at least one current collector (for example nickel mesh), which guides the electrons from the casing of the battery towards the catalyst, the air and the electrolyte, and also a catalyst which assists the transfer of electrons towards the oxygen which is reduced to form hydroxide, thus creating an electric current. Such a catalyst is for example an oxide of manganese.
The difficulty in producing the cathode of a metal/air battery originates from the fact that the catalyst must be in contact simultaneously with the current collector (solid), the air (gaseous) and the electrolyte (liquid). It is particularly complicated to succeed in making these three phases coexist on the greatest possible surface. In order to resolve this problem, one solution for increasing the interface between air, electrolyte and catalyst and therefore improving the power of the battery, consists in conferring a certain hydrophobicity to the cathode. The addition of hydrophobic additives in the composition of the cathode makes it possible to avoid the whole of the cathode being flooded by the electrolyte and thus leaving space for the air to penetrate better into the cathode. To this end, typically a hydrophobic binder is used, such as polytetrafluoroethylene (PTFE), in the form of powder or an aqueous dispersion. The addition of such a hydrophobic binder makes it possible to increase the power of the cathode as a function of the concentration of PTFE but only to an optimum concentration (typically 15-20%) beyond which the power of the battery decreases. This loss of power is due to the homogeneous dispersion of the PTFE in the entirety of the cathode which has the effect of reducing the electrical conductivity (interruption by PTFE of percolation of the network formed by the carbon particles) and of reducing likewise the quantity of electrolyte in the cathode.
Another solution for increasing the interface between air, electrolyte and catalyst consists of using a cathode which has a porous structure which makes it possible for the electrolyte and the air to penetrate better into the cathode. Typically, such a porous structure is obtained by using a mixture of various conductive particles, such as particles of conductive carbons (carbon black, graphite etc. . . . ).
Such a cathode is described for example in the patent application US 2014/0308594. According to this document, the cathode comprises an active layer which has a porous structure, the porosity of which decreases between the air side and the metal side of the cathode. Various manufacturing methods are described which make it possible to obtain an active layer which has a porosity gradient in the thickness of the active layer instead of a uniform porous structure. This porosity gradient is due to a variation in the intrinsic porosity of the material of the active layer of the cathode which is controlled during manufacture thereof.
The patent FR 2785093 likewise describes a cathode, the active layer of which has pores and comprises graphite particles, the average size of which is greater than the average diameter of the pores of the active layer. The cathode comprises furthermore a diffusion layer which is a hydrophobic membrane deposited on the air side of the active layer. This hydrophobic membrane is of the Gore-Tex® type which is made of expanded polytetrafluoroethylene (ePTFE). The cathode is obtained by preparing the active layer in the form of a paste, then the paste is spread over one face of the current collector. The membrane of the Gore-Tex® type is applied on the other face. Then the cathode is dried and compressed. In this cathode, the hydrophobic membrane, because of its structure, remains of a constant thickness, and the only role thereof is to make the surface of the cathode more hydrophobic in order to increase its resistance to flooding.
Metal/air batteries, essentially zinc/air, are currently used in auditory appliances because of their highest energy density among commercial batteries. For this application, the metal/air batteries must be replaced every week and are not therefore optimised with respect to their lifespan but are as it is optimised with respect to their power. However for applications such as connected watches, which involve very high current peaks for the primary batteries, it is necessary to have available batteries which have greater power. The power of metal/air batteries being limited by their cathode, it is necessary to develop cathodes which make it possible to obtain an increase in power of the battery which decreases only slightly in the course of time, in order thus to become useful in a clockmaking application.